Radiotherapy consists of projecting onto a predetermined region of a patient's body, a radiation beam so as to destroy or eliminate tumours existing therein. Such treatment is usually carried out periodically and repeatedly. At each medical intervention, the radiation source must be positioned with respect to the patient in order to irradiate the selected region with the highest possible accuracy to avoid radiating adjacent tissue on which radiation beams would be harmful.
When a tumour is located in the thoracic or abdominal chest cavities, the position of a tumour can vary considerably (e.g. by as much as a few centimeters) throughout the breathing cycle. In order to obtain reliable CT scanning data it is therefore important to obtain data consistently at the same point within a breathing cycle.
Certain modern CT scanners are able to process CT scanning data acquired at different times within a breathing cycle to generate a representation of the 3D movement of internal organs during breathing. Such “4D” representations of organ movement are invaluable in determining a course of treatment for irradiating a cancer. Again in order for quality planning data to be generated, the timing within the breathing cycle when individual CT scans are acquired must be known so that 4D representation accurately represents the movement of the internal organs.
When applying radiation to a patient, the gating of treatment apparatus should be matched with the breathing cycle so that radiation is focused on the location of a tumour and collateral damage to other tissues is minimised. If movement of a patient is detected the treatment should be halted to avoid irradiating areas of a patient other than a tumour location. Also if irregular breathing such as coughing is detected, treatment should be halted as such irregular breathing may cause a tumour to change location relative to the focus of a radiation beam.
For this reason a number of monitoring systems for monitoring a patient's breathing during radiotherapy have therefore been proposed. Thus for example, U.S. Pat. No. 6,621,889 discloses a system in which the gating of a radiation apparatus is linked to a determination that the periodicity of a patients breathing has deviated from an earlier measurement. Further examples are U.S. Pat. No. 6,980,679 and U.S. Pat. No. 7,123,758 which disclose a system for monitoring breathing of infants and reporting irregularities based on the periodicity of a patients breathing deviating from an earlier measurement.
An improved system for monitoring and measuring patient breathing and movement is, however desirable.